1. Field of the Invention
The present invention relates to a photoresist to be used for micro-fabrication of semiconductors, photomask blanks, and the like, and particularly to a chemically amplified positive or negative resist to be used for precisely conducting micro-fabrication by ultraviolet rays, EUV, electron beam exposure at wavelengths of 300 nm or shorter.
2. Description of the Related Art
It is known that finer pattern rules are demanded with highly integrated LSI's providing highly increased speeds. This has correspondingly led to largely changed exposure methods and resist compositions, and particularly, KrF, ArF excimer laser light, electron beam, and the like are used as exposure light sources upon conduction of lithography of patterns at 0.2 μm or less, in a manner to adopt chemically amplified photoresists, which exhibit excellent sensitivities to such high energy radiations and provide higher resolutions.
However, chemically amplified resist compositions are defectively accompanied by: a problem (called PED [Post Exposure Delay]) such that extended standing-still periods from exposure to PEB (post-exposure bake) lead to T-topped shapes of line patterns upon formation of positive patterns, i.e., lead to patterns having thickened upper portions; or a problem, which is a so-called footing phenomenon where patterns are thickened near a substrate, such as a substrate formed with metal like Al, Cr, or the like, or a basic substrate, particularly a silicon nitride substrate or titanium nitride substrate. The T-topped phenomenon is considered to be due to a decreased solubility of a resist film surface, while the footing at a substrate surface is considered to be due to a decreased solubility of the resist near the substrate.
In turn, upon formation of a negative pattern, there is caused a so-called undercut phenomenon where a cross-linking reaction of a negative resist is decreased near a substrate such that the pattern is subjected to constriction at a substrate-side boundary face of resist.
In case of chemically amplified positive resist compositions, the reason of the problem of PED or footing profile at a substrate surface is considered to be largely affected by a basic compound in air or at a substrate surface. Acids generated by exposure at a resist film surface react with basic compounds in air and are inactivated, such that longer standing-still times until PEB correspondingly increase amounts of acids to be inactivated, to scarcely cause decomposition of acid labile groups. As such, hardly soluble layers are formed at surfaces of resists, so that patterns are brought into T-topped shapes, respectively.
Against this problem, it is well known that addition of a nitrogen-containing compound allows for restriction of an influence of basic compounds in air, and this is also effective to PED (see Japanese Patent Application Laid-open (kokai) No. H05-232706, for example). Particularly, amine compounds or amide compounds have been noticed as nitrogen-containing compounds having higher addition effects, and numerous concrete compounds have been proposed.
Although it has been proposed to use a relatively weak base against the above-mentioned T-topped problem, the weak base is insufficient for control of deprotection reaction within a resist film, i.e., for control of diffusion of acids for causing a catalytic reaction, in case of adoption of acid labile groups having higher reactivities to be used for obtaining higher resolutions. Addition of weak base particularly results in progression of dark reaction in PED even at an non-exposed area, thereby causing a decrease of line dimension (slimming) and a film decrease at line surface in PED. To solve this problem, it is desirable to add a strong base. However, stronger basicities are not necessarily preferable, such that a sufficient effect can not be obtained even by addition of DBU (1,8-diazabicyclo[5.4.0]-7-undecene) or DBN (1,5-diazabicyclo[4.3.0]-5-nonene), which are regarded as being super base, or proton sponge (1,8-bis(dimethylamino)naphthalene), or quaternary ammonium hydroxides such as tetramethylammonium hydroxide.
Meanwhile, it is effective to add a nitrogen-containing compound having an improved effect for capturing generated acids, for increased contrasts for achieving higher resolutions. Although a dissociation constant between acid and base in water can be explained by pKa, an acid-capturing ability and pKa of a nitrogen-containing compound in a resist film are not directly related to each other. This has been stated by Hatakeyama et al., in a J. Hatakeyama, et. al., J. Photopolym. Sci. Technol., 13(4), 519-524 (2000). It has been additionally recognized that types of nitrogen-containing organic compounds to be used largely affect pattern profile.
Concerning pattern profile in substrate-side boundary face of resists, although problems of footing profile and undercut are improved by addition of base, the improvement is insufficient. While further increase of addition amounts of base will result in further improvement, there is caused a tradeoff problem of a considerably deteriorated sensitivity.
In turn, to solve a problem of footing profile of a resist pattern on a light-shielding film of a photomask upon fabrication of the light-shielding film, it has been disclosed that formation of a polymeric undercoat on a sputteredly deposited metal compound results in obtainment of a pattern without footing (Japanese Patent Application Laid-open (kokai) No. 2007-171520). However, the countermeasure on a substrate side requires an increased number of steps for processing, and is problematic not only in a complicated process but also in an increased production cost.
In turn, to improve footing on a Cr substrate of a mask blank, there has been proposed a resist composition combined with a basic additive including a base, which is solid at a room temperature (20 to 25° C.) and a low vapor pressure base, which is liquid at a room temperature (Japanese Patent Application Laid-open (kohyou) No. 2007-522524). Because production processes have been problematically complicated when footing is to be solved by the countermeasure on a substrate side, it is important that the problem is solved by a countermeasure on a resist composition side. However, adoption of the low vapor pressure base leads to evaporation of the base at a prebaking temperature, thereby occasionally causing a sensitivity change by a concentration change. This makes it difficult to keep dimensions of a pattern with higher precision.
Further, at an early stage of elaboration of chemically amplified resists during a proposal period of various basic compounds, there has been proposed addition of amino acid, 3-aminopyrazine-2-carboxylic acid, or the like, as a compound having an amino group and a carboxyl group (Japanese Patent Application Laid-open (kokai) No. H05-289340). However, there have not been found important proposals thereafter to use such an amine compound having a carboxyl group, and such a situation is considered to be due to failure of remarkable effects by such usage in seeking for a higher resolution.